The present invention relates to an incandescent lamp having tungsten filaments disposed in a bulb in which a halogen gas has been incorporated.
When tungsten filaments in incandescent lamps are heated to a temperature sufficient for the production of light tungsten sublimates from the filaments, the sublimated tungsten either remains suspended in the enclosed atmosphere of the lamp or is deposited on some other surface within the bulb. In small bulbs, the surface would normally blacken from the deposit of this sublimated tungsten. This blackening occurs so rapidly that the bulb would be of no practical use within a very short time.
It has been known for quite some time that the blackening of the bulb walls can be substantially reduced by adding a halogen to a standard inert atmosphere within the bulb, which may be any one or a mixture of inert gases such as argon, nitrogen, xenon, krypton, etc. With the halogen present, a minimum temperature on the order of 250.degree. C. or greater at the wall of the bulb will promote a reaction between the tungsten deposited on the wall and the halogen in the atmosphere. The resulting tungsten-halogen gas molecule eventually returns to the area of the filament. Because of the filament's high temperature, of the order of 1800.degree. C. to 2500.degree. C., the energy available to the molecule is sufficient to dissociate the tungsten-halogen molecule. Tungsten is redeposited on the filament and the halogen atom is released to repeat the process again. Thus, the highly desirable result of tungsten metal being removed from the walls of the bulb and returned to the filament from which it sublimated is obtained.
It has been found, however, that while the tungsten filament is being constantly benefited through the regenerative cycle, other metal parts in this atmosphere, including filament support legs and the like, are being attacked by the active halogen atoms in the same manner as the metal on the walls. And, since these other metal parts are not heated to the degree the filament is, there is no corresponding replacement of metal as occurs at the filament. This causes a weakening of such metal parts and considerably shortens the life of the bulb. Attempts have been made to ameliorate this undesirable result. For instance, U.S. Pat. No. 3,538,373 discloses the use of a carbon film on all metal parts in the bulb except for the filament to shield the metallic surface from attack by the halogen. U.S. Pat. No. 3,431,448 teaches the reduction of the attack on a filament by configuring the filament and/or supports in a manner to avoid a sharp temperature gradient in the filament wire. In U.S. Pat. No. 3,431,448 the sharp gradient is avoided either by increasing the pitch of the filament coil or by configuring the supports to act as a "heat sink" of gradually increasing efficiency.
For small halogen lamps of the type suitable for use in car headlamps, the filaments may be relatively small. Stretching the filament coil in a manner disclosed in U.S. Pat. No. 3,431,448 produces an unacceptable loss in efficiency, and in many bulbs there is simply no room in which to expand the extra filament length. Special heat-sinking supports are significantly more expensive to produce than the conventional wire supports.
The use of a coating may be highly desirable in numerous instances, but it is impossible to protect a second operating filament using this teaching. The high temperature of an operating filament simply vaporizes any coating which is applied. Also, with two filaments, one of which is non-operating during the time when the other filament is operating, each may be subject to an accelerated attack by the halogens because of their proximity to each other and because the non-operating filament is likely to be in the path of the convection of the highly activated halogen atoms being returned to the enclosed atmosphere from the operating filament.
It has now been discovered that an inoperative surrogate element positioned within the bulb will lessen the severity of the attack on the filament or other metal parts within the bulb. Furthermore, the use of such a surrogate element is not limited to such two-filament bulb and may be incorporated into single filament bulbs as well. The present invention is particularly important to the life of "halogen lamps" in which the filaments operate in an atmosphere that contains a halogen gas or a combined form of a halogen gas such as hydrogen bromide, methyl bromide or chloride carbon tetrabromide, etc. The term "halogen gas" used in the specification and claims is intended to mean halogen gas per se and those other gases in which halogen is present in a combined form as is conventional in the art for halogen lamps.